28o REDUCTION OF THE CHROMOSOMES 



Adelea (one of the Coccidiae) is a very interesting case, for accord- 

 ing to Siedlecki ('99) polar bodies or their analogues are formed in 

 both sexes. The gametes are here of very unequal size. Upon their 

 union the smaller male cell divides twice to form apparently equiva- 

 lent spermatozoids, of which, however, only one enters the ovum, while 

 three degenerate as polar bodies. These two divisions are of different 

 type ; the first resembles true mitosis, while the second is of simpler 

 character and is believed by Siedlecki to effect a reduction in the 

 number of chromosomes. In the meantime the nucleus of the macro- 

 gamete moves to the surface and there expels a portion of its chro- 

 matin, after which union of the nuclei takes place. Interesting facts 

 have been observed in unicellular plants which indicate that the 

 reduction may here occur either before (diatoms) or after (desmids) 

 fusion of the conjugating nuclei. In the iormtr {Rkopahdina) Klebahn 

 ('96) finds that each nucleus divides twice, as in many Infusoria, giving 

 rise to two large and two small nuclei. Each of the conjugates then 

 divides, each daughter-cell receiving one large and one small nucleus. 

 The four resulting individuals then conjugate, two and two, the large 

 nuclei fusing while the small (polar bodies) degenerate. The com- 

 parison of this case with that of the Infusoria is highly interesting. In 

 the desmids on the other hand (C/^i"/r/7V/;;/ and Cosmarmm, Fig. 140), 

 according to Klebahn ('92), the nuclei first unite to form a cleavage- 

 nucleus, after which the zygote divides into two. Each of the new 

 nuclei now divides, one of the products persisting as the perma- 

 nent nucleus, while the other degenerates and disappears. Chmie- 

 lewski asserts that a similar process occurs in Spirogyra. Although 

 the numerical relations of the chromosomes have not been determined 

 in these cases, it appears probable that the elimination of a nucleus in 

 each cell is a process of reduction occurring after fertilization. 



G. Maturation of Parthenogenetic Eggs 



The maturation of eggs that develop without fertilization is a sub- 

 ject of special interest, partly because of its bearing on the general 

 theory of fertilization, partly because it is here, as I believe, that one 

 of the strongest supports is found for the hypothesis of the individ- 

 uality of chromosomes. In an early article by Minot i^'J'j) on the 



a number (five to twelve) of " primary cysts," each containing one of the remaining nuclei. 

 Each primary cyst divides by mitosis to form two gametes ("secondary cysts"), which, after 

 forming the polar bodies, reunite, their nuclei fusing to form a single one. The resulting 

 cell soon creeps out of the cyst-wall and assumes the active life, its nucleus meanwhile mul- 

 tiplying to produce the multinuclear condition characteristic of the adult animal. What is 

 here the physiological motive for the formation of the polar bodies, and how shall it be 

 explained under the Weismann hypothesis? 



